Glow switch



. L. R. PETERS I GLOW SWITCH Oct. 26, 1943.

Filed March 26, 1941 Invervbor: L o RPe't'ers,

His At'tor-neg.

Patented Oct. 26, 1943? assascc oLow swrrcn Leo E. Peters, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application March 26, 1941, Serial No. 385,282

9 Claims.

, The present invention relates to thermal switches, and in particular to switches comprising a thermally responsive element actuated by heat generated by a gaseous electric discharge to make and break the interengagement of the switch contacts. Switches of this type are disclosed in the copending application Serial No. 289,987, filed August 12, 1939, inventor Wayne S. Smitley, which is assigned to the assignee of the present application. Such switches are now used commercially in the starting circuits of positive column discharge lamps having thermionic electrodes which require preliminary heating before starting the discharge in the lamp.

The switch in such circuits is used to connect the lamp electrodes in series across the terminals of an' alternating current source of commercial frequency and to shunt the discharge path between said electrodes until the electrodes have been heated to an electron emissive temperature. This preheating of the electrodes lowers the starting potential of the discharge lamp, avoids harmful ionic bombardment of the electrodes and premature termination of the useful life of the lamp. After the electrodes have been heated to the desired temperature, the switch breaks the series connection therebetween to interrupt the flow of current in the heater circuit and to impress an inductive voltage surge between the lamp electrodes to start the discharge. The inductance element for supplying the voltage surge to start the lamp is connected between a termi nal of the current source and an electrode of the lamp.

The switch comprises electrodes between which a heat generating electric discharge occurs, a thermally responsive element in heat receiving relation to the discharge and contacts constructed and arranged to be moved into and out of can gagement by said thermally responsive element. When a suitable potential is applied across the terminals of the switch the discharge starts therein and heats the thermally responsive element which closes the contacts. This extinguishes the discharge in the switch and permits a higher heating current to flow through the lamp electrodes. When the thermally responsive element cools sufficiently, it opens the contacts to interrupt .the heating current and to apply the inductive voltage surge, which is higher than the voltage of the current source, across the terminals of the lamp. The contacts remain in an open position during the operation of the lamp anduntil restarting of the lamp is desired.

Obviously, in such circuits the Voltage required to start the discharge in the switch (hereinafter called the breakdown voltage) must be lower than the voltage of the current source for the lamp and higher than the operating voltage of the lamp. 1' have observed that switches having the required breakdown voltage when the switch is exposed to light do not have such breakdown voltage when. the switch is in the dark. The absence of light causes the breakdown voltage of the switch to increase beyond the voltage of the current sources customarily used for such lamps and the switch is thus useless for starting the lamps in the dark.

An object of the present invention is to provide a thermal switch of the gaseous electric discharge type having a breakdown voltage which is substantially the same in the light and in the dark. Another object of the invention is to provide a method of manufacturing such switches. Other objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description thereof and from the appended claims.

I have discovered that the breakdown voltage 01 thermal switches of the gaseous electric discharge type is substantially the same in the light and in the dark when the switch comprises two electrically conducting paths in the switch en-' velope, one of said paths including the electrodes and the other of said paths connecting the cur-.

rent leading-in wires. The resistance of the electrically conducting path between the leadingin wires must be lower than that of the path including the electrode when the gas between said electrodes is not ionized, and higher than the resistance of the path including the electrodes when said gas is ionized. I have demonstrated that switches having the above structure have, in most instances, the same breakdown voltage in the light and in the dark and that, in all instances, the breakdown voltage of such switches in the dark is not more than approximately 10 volts higher than the breakdown voltage thereof in the light. Switches having the same structure as those embodying the invention, with the exception of the conducting path between the leading-in wires, and which have the same breakdown in the light as those embodying the invention, for example volts, do not start in the dark even when the voltage of the current source is approximately to volts. Prior switches having higher breakdown voltages in the light, such as a breakdown voltage of between to volts, and which do not have the conducting path between the leading-in wires, fail to start in the dark even when the voltage of the current source is as high as 210 volts.

I prefer to make the path between the wires in the form of a covering or coating of electrically conducting material over the inner surface ofthe envelope around and between the wires and connected to at least one of said wires. While many materials are useful for the covering. I have obtained excellent results with a covering consisting of powdered aluminum metal painted on the part of the inner surface of the envelope indicated above and on the part of the wires adjacent the innersurface of the envelope. This aluminum coating when first formed short-circuits the electrically conducting path between the electrodes of the switch, but the resistance thereof is increased by passing current of sufficient magnitude through said coating during the manufacture of the switch to disrupt it as in the blowing out of a fuse. The electrically conducting path through'the aluminum coating then has the desiredresistance and the breakdown voltage of the switch is substantially the same in the light and in the dark.

A species of a thermal glow switch of the gaseous electric discharge type embodying my invention is illustrated in the accompanying drawing in which Fig. 1 is a perspective view of the switch, Fig. 2 is a side elevational, fragmentary view of the stem part thereof, Fig. 3 is a front elevational view of said stem part, and Fig. 4 is an exploded view of the switch and a housing therefor.

The switch illustrated-in Figs. 1 to 3 of the drawing comprises a hermetically sealed glass envelope I having a reentrant stem 2 through which current leading-in wires 3 and 4 are hermetically fused or welded. The steam 2 is provided with an exhaust tube 5 through which the envelope I is exhausted of air and filled with the desired ionizable gaseous atmosphereduring the manufacture oftheswitch. The electrode 8 comprises a bimetallic element which is welded to support 8. such as a nickel rod, connected to the current lead 4. Said electrode I is bent over in a U-shape, as shown,-so that the free end thereof is located between support 8 and electrode Inear the top of the press of stem 2 and the lower part of theelongated electrode I. Said electrode I comprises a-metal rod, such as a nickel rod, connected to the current lead 3. A refractory metal contact 9, such as a rod of molybdenum, is welded to the electrode 8 adjacent or at the free end thereof and on the surface thereof facing the electrode 1. The contact 1 and the upper portion of electrode I may be about 0.5 to 0.8 mm. from electrode I. The envelope I has an ionizable gaseous atmosphere therein. such as argon at a pressure of about 25 mm., helium at a pressure of about 100 mm., or a mixture of helium and 0.01 to 5 per cent argon at a pressure of-iabout 100 mm.

The electrode I comprises two strips of metal having diiferent coefficients of linear expansion securely fastened together, as by welding. The inner strip has a greater coefficient of expansion than the outer strip and the free end of the electrode I moves toward the electrode 1 with increasing temperature and away from the electrode I with decreasing temperature. While I prefer to make the inner strip of-a chrome iron alloy and the outer strip of a nickel iron alloy, itwill be understood, of course, that other metals or alloys having the desired characteristics may be used when desired.

Th electrode 8 and the ments has a coating II thereon of electrically conducting material, such as aluminum. Said coating III also covers the lower portion of electrode I and the support 3 adjacent the surface of the stem press. The coating I0 is applied to these parts by painting with a mixture of aluminum metal powder, amyl acetate and nitrocellulose lacquer after the electrode 8 is mounted on its support and before the stem 2 is mounted in the envelope .I. The paint should completely cover the surface of the stem press between and around the electrode I and the support 6. A comparatively thin coating is preferred since this dries quickly to facilitate manufacture of the switch. A paint consisting of grams of aluminum metal powder capable of being washed through a 325 mesh screen, 25 cubic centimeters of nitrocellulose lacquer and 190 cubic centimeters of amyl acetate is satisfactory for this purpose. I

The painted stem} is fused into the envelope I after the coating I. has dried. The envelope I is then exhausted of air and filled with the desired gaseous atmosphere through the exhaust tube II on the stem 2 after which the tube I I is tipped off in the usual manner to hermetically seal the envelope I,

The coating in forms an electrically conducting path having a lower resistance than the ioniged gas in the discharge path between the electrodes I and 8 at this stage of the manufacture. This condition prevents the functioning of the glow switch and is corrected by passing suillcient current therethrough after the envelope I has been hermetically sealed to disrupt the continuous conducting path through the coating III. The break in the coating usually occurs adjacent one of the inleads. Immediately on the rupture of the coating III, a glow discharge starts between electrodes I and 8 which bombards the coating of zinc on the electrode 8 and sputters it onto the internal parts of the switch including the electrode 1. Thus the electrodes I and 8 comprise a metal support covered with a metal of lower work function, i. e., a metal of high elec tron emissivity. The glow discharge is continued until the switch has the desired breakdown potential. The switch is then ready for use in the lamp circuit.

Switches having the above structure and containing helium at a pressure of about mm., or the other gas or gas mixtures at the pressures disclosed above, have a breakdown voltage of between and 1'10 volts and successfully operate in the dark and in the light in lamp circuits when the voltage of the alternating current source for the lamp is as low as approximately volts and the lamp operating voltage is as high as approximately 120 volts. Other gases or gas mixtures and other electrode coating materials may be used to obtain other breakdown potentials, when desired. For example, a switch similar to that described above having magneslum on the electrodes and a gaseous atmosphere consisting of a mixture of neon and about 0.1 to 5 per cent argon at a pressure of approximately 40 mm. has a breakdown potential of as low as '75 volts. Such a switch is useful for starting lamps in the light and in the dark in circuits in which the voltage of the alternating current source is approximately 110 to 120 volts and lower and the operating voltage of the lamp approximately 65 volts.

While I prefer a coating of finely divided aluminum on the stem; I have demonstrated that similar coatings of other electrically conducting materials, such as bronze and carbon, are effective for the purpose of the invention. I have demonstrated further that a coating of zinc is efiective when the electrodes have Zinc thereon and that a coating of magnesium is effective when the electrodes have magnesium thereon. Other methods of applying the coating, such as spraying, vaporizing or sputtering, may be .used when desired. A cap of electrically conducting material, such as nickel, covering the parts of the stem between and about the inleads, electrically connected to one inlead and separated from the other inlead by a gap is also effective for the purpose of the invention.

When a suitable potential is impressed across the terminals of the switches described above, a glow discharge starts between the electrodes 1 and 8. -'I'he heat of the discharge causes the electrode 8 to flex and thus to move the contact 9 toward and against the electrode 1 'to short cirsuit the discharge between the electrodes 1 and 8. A heater current of sufllcient magnitude to raise the lamp electrodes to an arc dischargesustalning temperature then flows through the metal parts of the switch. The bimetallic elec-.

trode 8 cools rapidly after the discharge incident thereat has been shortcircuited and separates the contact 9 from the electrode 1 to interrupt the flow of said heater current. Simultaneously an inductive voltage surge is applied across the terminals of the lamp to start the discharge therein. The potential impressed across-the terminals of the switch during the operation of the lamp is not sufficient to start a discharge between the electrodes 1 and 8 and the switch thus does not operate during the operation of the lamp. If, however, the lamp fails to start when one cycle of operation of the switch has been completed, the switch continues its cycle of operation until ignition of the lamp is accomplished.

I have illustrated in Fig. 4 a lamp starting unit incorporating my improved switch and a container or can I! for supporting the switch.

The container I2 is open at one end only and 7 consists of metal, such as aluminum. A paper sleeve I3 is inserted in said container l2. A disc ll of opaque insulating material having a spaced pair of electrical contacts [9 thereon mechanically closes the open end ofv container l2 in the assembled unit. The leading-in wires 3 and 4' of the switch and the wires of a condenser I1 are fastened to said contacts 19. One of said contacts.|9 and a wire l5 of the condenser I have been shown in the drawing for simplicity of iilustration. The condenser ll prevents radio interference by the switch. In the assembled unit the switch and the condenser H are surrounded by the sleeve I3 and all of these elements are supported in the container l2 by the disc ll which is clamped against the open end of the container I! by the ears l8 on said container. When the switch unit is mounted in the auxiliary apparatus for the lamp, each of the contacts 19 is electrically connected to a terminal of one of the lamp electrodes.

What I claim as new anddesire to secure by Letters Patent 01 the United States is:

i. A thermal switch of the gaseous electric discharge type comprising a sealed envelope containing an ionizable gaseous atmosphere and 00- operating electrodes, electrical conductors passing through the wall of said envelope and electrically conducting material in said envelope between said conductors and providing a path for the passage of electrical energy therebetween, the resistance of said path being higher than that of the electrically conducting path through the gas and said electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

2. A thermal switch of the gaseous electric discharge type comprising a sealed envelope con-- taining an ionizable gaseous atmosphere and cooperating electrodes, electrical conductors passing through the wall of said envelope and a covering of electrically conducting material for the inner surface of said envelope between said conductors, said electrodes being secured to said conductors, said covering providing a path for the passage of electrical energy between said conductors, the resistance of said path being higher than that of the electrically conducting path through the gas and said electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

3. A thermal switch of the gaseous electric discharge type comprising a sealed envelope con taining an ionizable gaseous atmosphere and cooperating electrodes, electrical conductors passing through thewall of said envelope and a coating of finely divided aluminum on the inner surface of said envelope between said conductors, said coating providing a path for the passage of electrical energy between said conductors, the resistance of said path being higher than that of the electrically conductingpath through the gas said electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

4. A thermal switch of the gaseous electric discharge type comprising a sealed envelope containing an ionizable gaseous atmosphere and having a reentrant stem, current leading-in wires fused to and passing through said stem into said envelope, cooperating electrodes connected to said wires in said envelope and a covering of electrically conducting material for said stem between said wires to provide a shuntpath in said envelope for the passage of electrical energy between said wires, the resistance of said shunt path being higher than that 01' the path through the gas between said electrodes aiter said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

5. A thermal switch of the gaseous electric discharge type comprising a sealed envelope containing an ionizable gaseous atmosphere and having a reentrant stem, current leading-in wires fused to and passing through said stem into said envelope, cooperating electrodes connected to said wires in said envelope and a covering of aluminum for said stem between said wires to provide a shunt path in said envelope for the passage of electrical energy between said wires, the resistance or said shunt path being higher than that of the path through the gas between said electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

6. A thermal switch of the gaseous electric discharge type comprising a sealed envelope containing an ionizable gaseous atmosphere and cooperating electrodes, one of said electrodes comprising a metal support 01' an iron alloy and a quantity of zinc on said support, electrical conductors passing through the wall of said envelope and a covering of electrically conducting material for the inner surface of said envelope between said conductors, said electrodes being secured to said conductors, said covering providing a path for the passage of electrical energy between said conductors, the resistance of said path being higher than that of the electrically conducting path through the gas and said-electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

7. A thermal switch of the gaseous electric discharge type comprising a sealed envelope containing an ionizable gaseous atmosphere and cooperating electrodes, one of said electrodes comprising a metal support of an iron alloy and a quantity of magnesium on said support, electrical conductors passing through the wallet said envelope and a covering of electrically conducting material for the inner surface of said envelope between said conductors, said electrodes being secured to said conductors, said covering providing a path for the passage of electrical en- 'ergy between said conductors, the resistance of said path being higher than that of the electrically conducting path through the gas and said electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient'radiation.

8. A thermal switch of the gaseous electric discharge type comprising a sealed envelope containing an ionizable gaseous atmosphere and cooperating electrodes, one of said electrodes comprising a U-shaped bimetallic body and a metal contact member mounted on said U-shaped body, said U-shaped body being capableof moving said contact member toward the other of said electrodes in response to increasing temperature to short circuit the discharge path between said electrodes, electrical conductors passing through the wall of said envelope and a covering of electrically conducting material for the inner surface of said envelope between said conductors, said electrodes being secured .to said conductors, said covering providing a path for the passage of electrical energy between said conductors, the resistance of said path being higher than that 01' the electrically conducting path through the gas and said electrodes after said gas is ionized and lower than the resistance of said last named path before said gas is ionized whereby the breakdown voltage of the switch is substantially independent of ambient radiation.

9.'In combination, a thermal switch of the gaseous electric discharge type and an opaque enclosure therefor, said switch comprising a sealed envelope containing an ionizable gaseous atmosphere and cooperating electrodes, electrical conductors passing through the wall of said envelope and a covering of electrically conducting material for the inner surface of said en- LEO R. PETERS. 

